The present invention relates to gamma cameras and more particularly to a method for predicting failure of photomultiplier tubes (PMTs) prior to failure occurring.
Single photon emission computed tomography (SPECT) examinations are carried out by injecting a dilution marker comprising a compound labeled with a radiopharmaceutical into the body of a patient to be examined. A radiopharmaceutical is a substance that emits photons at one or more energy levels. By choosing a compound that will accumulate in an organ to be imaged, compound concentration, and hence radiopharmaceutical concentration, can be substantially limited to an organ of interest. A radiopharmaceutical that emits photons or gamma emissions which are approximately at a single known energy level is chosen. The organ to be imaged will be referred to as an organ of interest and an energy range which approximates the known energy level will be referred to as the marker range.
While moving through a patient's blood stream the marker, including the radiopharmaceutical, becomes concentrated in the organ of interest. By measuring the number of photons emitted from the organ of interest which are within the marker range, organ characteristics, including irregularities, can be identified.
To measure the number of emitted photons, one or more planar gamma cameras are used. After a marker has become concentrated within organ of interest, a camera is positioned at an imaging angle with respect to the organ of interest such that the organ is positioned within the camera's field of view FOV. The camera is designed to detect photons traveling along preferred paths within the FOV.
A gamma camera consists of a collimator, a scintillation crystal, a plurality of photo multiplier tubes (PMTs) and a camera processor. The collimator typically includes a rectangular lead block having a width dimension and a length dimension which together define the FOV. The collimator block forms tiny holes which pass therethrough defining the preferred photon paths. The preferred paths are unidirectional and perpendicular to the length of the collimator. The collimator blocks emissions toward the crystal along non-preferred paths.
The scintillation crystal is positioned adjacent the collimator on a side opposite the FOV and has an impact surface and an oppositely facing emitter surface. The impact surface defines a two dimensional imaging area A having a length L and a width W. Photons which pass through the collimator impact and are absorbed by the impact surface at impact points. The crystal emitter surface emits light from an emitter point adjacent the impact point each time a photon is absorbed. The amount of light emitted depends on the absorbed photon's energy level.
The PMTs typically include between 37 and 91 PMTs which are arranged in a two dimensional array which is positioned adjacent the emitter surface. Light emitted by the crystal is detected by the PMTs which are in the area adjacent the emitter point. Each PMT which detects light generates an analog intensity signal S.sub.i. Intensity signal is proportional to the amount of light detected S.sub.i and a PMT intrinsic gain factor G.sub.i. When a single photon is absorbed by the crystal, the emitted light is typically absorbed by several different PMTs such that several PMTs generate intensity signals simultaneously.
A separate autotuner is associated with each PMT and includes, among other things, an autotuner amplifier. The autotuner amplifier receives the intensity signal from an associated PMT and steps the signal up by an autotuner amplifier gain G.sub.a which is unique to the amplifier (i.e. each autotune amplifier is uniquely tuned) thereby generating an autotuner amplifier signal S.sub.a. Each autotuner amplifier includes hardware for facilitating an autotuning procedure as will be described in more detail below.
A single high gain amplifier receives each autotuner amplifier signal S.sub.a and steps up the received signal S.sub.a by a high voltage gain G.sub.hv thereby generating a separate final signal S.sub.f for each PMT. Thus, each final signal S.sub.f generated by the high voltage amplifier can be expressed as: EQU S.sub.f =S.sub.i .times.G.sub.i .times.G.sub.a .times.G.sub.hv(1)
where S.sub.i is the initial detected light, G.sub.i is the intrinsic PMT gain, G.sub.a is the autotune amplifier gain and G.sub.hv is the high voltage amplifier gain. For the purposes of this explanation all final signals S.sub.f caused by a single photon will be collectively referred to as a signal set.
The processor receives each signal set and performs a plurality of calculations on each signal set to determine two characteristics of the corresponding photon. First, the processor combines the final signals S.sub.f of each signal set to identify the energy level of a corresponding photon. Photons having energies within the marker range will be referred to as events. Only signals corresponding to events are used for imaging. Second, the processor performs a series of calculations in an effort to determine precisely where on the impact surface imaging area A an event occurred. Once impact locations of all events have been identified, the processor uses the impact locations to create an image of the organ of interest which corresponds to the camera imaging angle.
To create a three dimensional image of the organ of interest, a gamma camera can be used to generate a plurality of images from different imaging angles. To this end, the camera is positioned parallel to, and at an imaging angle about, a rotation axis which passes through the organ of interest. The angle is incremented between views so that the plurality of images are generated. The plurality of images are then used to construct pictures of transaxial slices of the torso section using algorithms and iterative methods that are well known to those skilled in the tomographic arts.
With any imaging system there are several different criteria by which to judge system usefulness. One of the most important criteria for judging system usefulness is the quality of resulting images. As an initial matter, to generate a quality image, PMTs and associated amplifiers have to be capable of, given a specific intensity radiation source, generating a final signal for imaging having a known target value S.sub.th. This requirement is discussed in more detail below.
Also, to a great extent, image quality depends on camera PMTs operating in predictable fashion. For example, assume an organ of interest is positioned in a camera FOV during a first imaging period and a first image is generated and used for diagnosis. If, during a subsequent imaging period, the same patient is positioned in the same FOV and a second image is generated, if one of the camera PMTs operates differently than it did during the first imaging period an unexpected image artifact may appear in the second image which effectively reduces the diagnostic value of the second image.
Unfortunately, despite recent improvements in PMT manufacturing, nearly all PMTs are characterized by decreasing intrinsic gains G.sub.i, over their useful life. Thus, when a PMT is initially used its intrinsic gain G.sub.i is often relatively high but, over time, gain G.sub.i decreases appreciably. Complicating matters further, the rate at which intrinsic gain G.sub.i decreases is PMT specific (i.e. the rate of gain change is different between even identically constructed PMTs).
To compensate for decreasing intrinsic gains G.sub.i, the industry has developed various PMT tuning processes. According to most tuning processes, prior to initially using a camera, a known light source is provided to each PMT. To this end often a uniform radiation source is provided in the camera FOV. Because the source is uniform, the crystal provides a uniform quantum of light to each PMT. In the alternative, some PMTs include a built in LED or other type of light source of known intensity between the crystal and the PMT. The LED is only turned on during autotuning. When the light source is provided a final signal S.sub.f (i.e. high voltage amplifier output signal) for each PMT is recorded.
Because light source intensity is known, a target value S.sub.th for the final signal is also known. During the tuning process, where the measured final signal S.sub.f is less than target value S.sub.th, the autotune amplifier gain G.sub.a (see Equation 1) is increased thereby increasing the value of the final signal. Where the measured final signal S.sub.f is greater than target value S.sub.th, gain G.sub.a (see Equation 1) is decreased thereby decreasing the value of final signal S.sub.f until it equals target value S.sub.th.
During routine subsequent tuning processes, the known light source (e.g. flood or LED) is again provided to each PMT generating a final signal S.sub.f for each PMT. The final signal S.sub.f is again compared to target value S.sub.th for each PMT and each PMT autotuner amplifier gain G.sub.a is modified until an associated final signal S.sub.f is equal to target value S.sub.th. Typically, intrinsic gains G.sub.i decrease over time and therefore gains G.sub.a are usually increased during the tuning process. Autotuner amplifier gains G.sub.a are stored and used later during normal diagnostic imaging.
While autotuning improves detector stability and extends the time period between required service calls, autotuning is only effective up to the point where amplifier gain G.sub.a is at its maximum value. Once gain G.sub.a has been increased to its maximum value, any further PMT degradation cannot be compensated by using the autotuner amplifier and an event referred to as autotune failure occurs. At this point an autotuner will typically indicate failure via an alarm.
Once any one of the autotune amplifier gains G.sub.a has been increased to its maximum gain value to compensate for PMT degradation, one way to increase PMT life even further is to increase high voltage gain G.sub.hv. Referring again to Equation 1, during autotuning, assuming autotuner amplifier gain G.sub.a, is at its maximum level and the final signal S.sub.f is still below target value S.sub.th, by increasing high voltage gain G.sub.hv final signal S.sub.f can be increased further and until final signal S.sub.f is equal to target value S.sub.th.
High voltage gain G.sub.hv is not PMT specific but rather is identical for all PMTs in a camera. Therefore, when gain G.sub.hv is increased to extend the life of one PMT, gain G.sub.hv is also increased for all other PMTs in an array. Referring yet again to Equation 1, when gain G.sub.hv is increased to extend the life of a first PMT but does not have to be increased to extend the life a second PMT, to compensate for increasing gain G.sub.hv in Equation 1 for the second PMT, autotuner amplifier gain G.sub.a is decreased in a manner similar to the increase in gain G.sub.hv. For example, if G.sub.hv is increased by 10% to extend the life of the first PMT, the autotuner amplifier gain G.sub.a for the second PMT might be decreased by 10% (or some other suitable amount depending on how gains G.sub.a and G.sub.hv are implemented).
Despite gain G.sub.a and G.sub.hv modifications, over time, as the intrinsic PMT gain decreases, eventually a point is reached where, for some PMT within an array, both the autotuner amplifier gain G.sub.a and the high voltage gain G.sub.hv are at their maximum values. At this point, any further loss in intrinsic gain cannot be compensated for and an error as described above will result in an image produced using the PMT characterized by low intrinsic gain G.sub.i. This event is referred to hereinafter as complete failure.
When complete failure (i.e. G.sub.amax G.sub.hvmax) occurs and the camera cannot be used to generate precise images required for diagnostic purposes until the failed PMT is replaced. To this end, a technician or maintenance person must be notified and the PMT replaced.
Manual replacement of PMTs is disadvantageous for a number of reasons. First, as most medical facilities do not have in house imaging technicians, it may take hours or even days for a technician to replace a failed PMT. Not only is this a burden to patients who might have to reschedule imaging time but it is also expensive for a medical facility as imaging systems are expensive and therefore should be used as much as possible to justify their costs. Moreover, when a PMT is replaced, recallibration can be a relatively lengthy process resulting in further down time.
In addition, presently PMTs are only replaced after they fail and the failure signal is generated. Thus, while several PMTs may be near failure when a technician visits to replace a single PMT, the technician will only replace the single failed PMT.
To be more efficient, when a technician visits to replace one PMT, the technician might also replace other PMTs associated with autotune amplifiers which are at or near their maximum amplifier gains G.sub.a when high voltage gain G.sub.hv is near or at its maximum value.
Unfortunately, there is no good way to determine how close a specific PMT is to failure based solely on the instantaneous high voltage gain G.sub.hv and an autotuner amplifier gain G.sub.a. This is because, as indicated earlier, even identically manufactured PMTs may deteriorate at different rates. Therefore, when high voltage gain G.sub.hv is near its maximum value, while one PMT having a maxed out autotuner amplifier gain G.sub.a may be near failure, another PMT having a maxed out gain G.sub.a may still have many hours of or even days of useful imaging life left.
Therefore, it would be advantageous to have a method for accurately predicting when a PMT is approaching failure so that PMTs can be replaced as close to their failure points as possible resulting in more efficient systems and less system down time.